Process for recovery of metal values from materials containing arsenic and/or antimony

ABSTRACT

A method for the recovery of metal values from a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions such as sulphide ore or concentrate is disclosed. In one form the method comprises the steps of: (a) providing a feed stream comprising a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions; (b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions in the presence of at least one component selected to decrease the effective concentration of free acid generated during the pressure oxidation step and promote the formation of pH-stable iron (III) sulphate products, thereby forming a slurry comprising a metal value-containing leach solution and a solid leach residue containing pH-stable iron (III) sulphate products and environmentally stable iron-arsenic and/or iron-antimony products; (c) separating the metal value-containing leach solution from the solid leach residue; (d) recovering the metal value (s) from the metal value-containing leach solution; and (e) recovering precious metal values, if present, in the solid leach residue by cyanide leaching.

FIELD OF THE INVENTION

The present invention relates to a pressure oxidation process for therecovery of metal values, in particular copper and optionally gold andother precious metal values, from metal value-bearing materialscontaining arsenic and/or antimony.

The present invention relates more particularly but not exclusively to aprocess of sequestering arsenic and/or antimony in a stable iron-arsenicand/or iron-antimony compound or compounds from a metal value-bearingmaterial containing arsenic and/or antimony.

The present invention also relates more particularly but not exclusivelyto a process by which the recovery of the metal values can be increasedby enhancing the formation of a stable iron-arsenic and/or iron-antimonycompound or compounds from a metal value-bearing material containingarsenic and/or antimony.

The present invention also relates more particularly but not exclusivelyto a process by which recovery of metal values can be enhanced byinclusion of a post-pressure oxidation digestion-conditioning step priorto solid/liquid separation and recovery of metal values, in particularcopper and optionally gold and other precious metal values, from theseparated solid and/or liquid phases.

The present invention also relates more particularly but not exclusivelyto a process by which recovery of metal values, particularly gold andother precious metals, can be enhanced by means of promotion of theformation of pH-stable pressure oxidation residues for efficient metalrecovery through manipulation of the pressure oxidation solutionchemistry to limit free acid levels and/or promote the precipitation andstability of an iron-containing residue, particularly jarosite.

BACKGROUND OF THE INVENTION

In this specification, where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date:

-   -   (i) part of common general knowledge; or    -   (ii) known to be relevant to an attempt to solve any problem        with which this specification is concerned.

Many base metals are sourced from sulphide ores. For example, coppersulphide minerals such as chalcopyrite [CuFeS₂] contribute to themajority of global copper production. There are also many deposits thatcontain copper in the form of arsenic-bearing minerals, primarilyenargite [Cu₃AsS₄] and tennantite [Cu₁₂As₄S₁₃], and/or antimony-bearingminerals such as tetrahedrite [Cu₁₂Sb₄S₁₃].

Any process for recovery of metal values from metal sulphide-bearingmaterials that also contain arsenic requires consideration of the formto which the arsenic component reports and the environmental impact uponits disposal.

Pyrometallurgical treatment of metal-bearing sulphide materials thatalso contain arsenic is generally regarded as technically andeconomically undesirable, as most of the arsenic reports as flue dustand a speiss phase. Safe disposal of these arsenic-containing materialsinvolves considerable cost and technical disincentives.

By contrast, many hydrometallurgical processes for treating coppersulphide materials that also contain arsenic are directed towards thegeneration of an acidic sulphate solution containing soluble copper,which is typically recovered therefrom by a combination of solventextraction and electrowinning. The arsenic component of the feedmaterial is converted into an insoluble arsenic-containing phase such asscorodite [FeAsO₄.2H₂O]. This arsenic-containing phase can be safelydisposed of in a conventional tailings impoundment. The copper leachingstage may be conducted under oxidizing conditions at a temperature aboveambient and a pressure above atmospheric in a process commonly referredto as pressure oxidation.

The kinetics of the copper leaching stage of such a treatment arefrequently slow and there is generally coprecipitation of aniron-copper-arsenate-sulphate compound or compounds, leading to copperlosses to the leach stage solid residue and thus to the overall process.

Many copper sulphide materials that contain arsenic and/or antimonyoften also contain precious metals (gold and silver) and any process totreat the copper-arsenic-antimony sulphide material must also employeconomically viable treatment stages to recover the precious metalcontent. In the pressure oxidation process described above, the preciousmetals generally report to the solid residue generated by the leachprocess. These precious metals are usually recovered by re-pulping andcyanidation. The presence of meta-stable solid iron compounds such asbasic ferric sulphate [Fe(OH)SO₄] and any copper-containing precipitatessuch as an iron-copper-arsenate-sulphate in the residue decompose (breakdown) under the alkaline pH conditions required for gold/silvercyanidation leaching and as a result increase lime and cyanideconsumption, decreasing the economic efficiencies of the overallprocess. In other words, the abovementioned solid components present inthe leach residue break down during the cyanidation step, generatingexcess acid and reactive sulphate compounds that must be subsequentlyneutralised. The breakdown of the abovementioned solid components underalkaline conditions may also release copper and other metals in a formwhich is reactive with the cyanide ion, thereby increasing cyanideconsumption.

In summary, many of the hydrometallurgical processes currently employedto treat arsenic- and/or antimony-containing copper sulphide materialssuffer from unacceptable copper losses to the leach residue. Moreover,if the feed materials also contain precious metals, then currentprocessing conditions also lead to the generation of solid residues thatresult in unacceptably high lime and cyanide consumption.

The present invention seeks to overcome at least some of theaforementioned disadvantages.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod for the recovery of metal values from a metal value-bearingmaterial containing arsenic and/or antimony and a source of sulphateions such as sulphide ore or concentrate, the method comprising thesteps of:

-   -   (a) providing a feed stream comprising a metal value-bearing        material containing arsenic and/or antimony and a source of        sulphate ions;    -   (b) subjecting the feed stream to oxidative conditions under        elevated temperature and pressure conditions in the presence of        at least one component selected to decrease the effective        concentration of free acid generated during the pressure        oxidation step and/or promote the formation of pH-stable        iron(III) sulphate products, thereby forming a slurry comprising        a metal value-containing leach solution and a solid leach        residue containing pH-stable iron(III) sulphate products and        environmentally stable iron-arsenic and/or iron-antimony        products;    -   (c) separating the metal value-containing leach solution from        the solid leach residue;    -   (d) recovering the metal value(s) from the metal        value-containing leach solution; and    -   (e) recovering precious metal values, if present, in the solid        leach residue by cyanide leaching.

The slurry from step (b) may be maintained at a temperature in the rangeof from about 70° C. to about 100° C. for a period in the range of fromabout 15 minutes to about 4 hours prior to separating the metalvalue-containing solution from the solid leach residue.

According to another aspect of the present invention there is provided amethod for the recovery of metal values from a metal value-containingfeed material containing arsenic and/or antimony and a source ofsulphate ions such as a sulphide ore or concentrate, the methodcomprising the steps of:

-   -   (a) providing a feed stream of metal value-containing material        containing arsenic and/or antimony and a source of sulphate        ions;    -   (b) subjecting the feed stream to acidic oxidative conditions        under elevated temperature and pressure conditions in the        presence of at least one iron-containing compound and/or at        least one chemical agent, wherein the iron-containing compound        and/or chemical agent is selected to decrease the effective free        acid concentration generated during the pressure oxidation step        and/or promote the formation of pH-stable iron(III) sulphate        products, thereby forming a metal value-containing solution and        a solid leach residue containing pH-stable iron(III) sulphate        products and environmentally stable iron-arsenic and/or        iron-antimony products;    -   (c) separating the metal value-containing solution from the        solid leach residue;    -   (d) recovering the metal value(s), if desired, from the metal        value-containing solution; and    -   (e) recovering precious metal values, if present, in the solid        leach residue by cyanide leaching.

The metal value-containing solution and solid leach residue obtainedfrom step (b) may be maintained at a temperature in the range of fromabout 70° C. to about 100° C. for a period in the range of from about 15minutes to about 4 hours prior to separating step (c).

According to another aspect of the present invention there is provided aprocess for recovery of metal values from a metal value-bearing materialcontaining arsenic and/or antimony and a source of sulphate ions such assulphide ore or concentrate, the process comprising the following majorsteps of:

-   -   (a) providing a feed stream comprising a metal value-bearing        material containing arsenic and/or antimony and a source of        sulphate ions;    -   (b) subjecting the feed stream to oxidative conditions under        elevated temperature and pressure conditions in the presence of        at least one iron-containing compound and/or at least one        chemical agent, wherein the iron-containing compound and/or        chemical agent is selected to decrease the effective        concentration of free acid generated during the pressure        oxidation step and/or promote the formation of pH-stable        iron(III) sulphate products, thereby forming a slurry comprising        a metal value-containing leach solution and a solid leach        residue containing pH-stable iron(III) sulphate products and        environmentally stable iron-arsenic and/or iron-antimony        products;    -   (c) maintaining the slurry obtained from step (b) at a        temperature in the range of from about 70° C. to about 100° C.        for a period in the range of from about 15 minutes to about 4        hours;    -   (d) separating the metal value-containing leach solution from        the solid leach residue;    -   (e) recovering the metal value(s) from the metal        value-containing leach solution; and    -   (f) recovering precious metal values, if present, in the solid        leach residue by cyanide leaching.

According to another aspect of the invention there is provided a methodfor recovering one or more metal values from a metal value-containingfeed material containing arsenic and/or antimony comprising the stepsof:

-   -   (a) providing the feed material to a pressure oxidation reactor;    -   (b) subjecting the feed material within the pressure oxidation        reactor to pressure oxidation in the presence of at least one        component selected to decrease the amount of free acid generated        during the pressure oxidation and/or promote the formation of        pH-stable iron(III) sulphate products, to form a slurry        comprising:        -   (i) a metal value-containing solution; and        -   (ii) a solid leach residue comprising the pH-stable            iron(III) sulphate products and environmentally stable            iron-arsenic and/or iron-antimony products;    -   (c) separating the metal value-containing solution from the        solid leach residue; and    -   (d) recovering the metal values from the metal value-containing        solution.

According to another aspect of the invention there is provided apressure oxidation process for recovering one or more metal values froma metal value-bearing material containing arsenic and/or antimony, theprocess comprising the step of providing to the process at least onecomponent selected to promote the formation of solids that aresufficiently stable under pressure oxidation conditions such that therecovery of the one or more metal values is enhanced.

The terms “pressure oxidation” or “pressure oxidation step” or“oxidative conditions under elevated temperature and pressure” as usedherein refer to a high temperature/high pressure leach process operatedunder acidic oxidising conditions.

One particular aspect of the present invention is based upon therealisation that it is possible to adjust the processing conditions suchthat they limit the formation of insoluble copper-containingprecipitates during the high temperature pressure leaching process toextract metal values such as copper from a metal value-bearing materialsuch as a sulphide ore or concentrate that also contains arsenic and/orantimony.

Another particular aspect of the present invention is based upon therealisation that it is possible to adjust the processing conditions thatpromote the formation of solid iron(III) sulphate containing-products inthe residue derived from the pressure leaching process that are stableunder the alkaline pH conditions at ambient temperature that are used torecover the precious metals from said residue by cyanidation. Forconvenience, this solid iron(III) sulphate containing-product isreferred to as a “pH stable iron(III) sulphate”. The result of thecorrect selection of the high temperature pressure leaching conditionsfor treating metal value-bearing materials containing arsenic and/orantimony is that the majority of the arsenic and/or antimony reports toa solid residue as an environmentally stable mixed iron-arsenic and/oriron-antimony solid species mixed with pH-stable iron(III) sulphateproducts.

In addition, copper losses to the residue are minimised by limiting theprecipitation of a copper-iron-sulphate-arsenate, while cyanidation ofthe precious metals content of the leach residue is enhanced because ofthe promotion of precipitation of pH-stable iron(III) sulphate productssuch as jarosite-type minerals rather than basic iron sulphate.

The present invention is accordingly concerned with the development ofeconomically viable conditions that can at least partially achieve oneor more of (a) minimising copper losses to the leach residue, (b)ensuring that the arsenic and/or antimony components of the feedmaterial report to the residue in an environmentally stable form, and(c) preventing the formation of solid residues that break down duringthe precious metal cyanidation step and a concomitant increase in limeand cyanide consumption in the case where the initial feed materialcontains recoverable precious metals.

Preferably, the metal value-bearing material containing arsenic and/orantimony is a copper-bearing material containing arsenic and/orantimony, in particular a copper sulphide containing arsenic and/orantimony, and even more preferably a mixed copper-gold sulphidecontaining arsenic and/or antimony. Typically the metal value-containingfeed material is an ore or concentrate that contains arsenic and/orantimony, and includes but is not limited to:

-   -   (a) an ore or concentrate that contains recoverable base and        other metals including but not limited to copper, nickel,        cobalt, zinc and the platinum group metals;    -   (b) an ore or concentrate that contains precious metals        recoverable by cyanidation, especially gold and silver; and    -   (c) an ore or concentrate that contains recoverable base and        other metals including but not limited to copper, nickel,        cobalt, zinc and the platinum group metals, as well as        recoverable precious metals, especially gold and silver.

Typically, the pH-stable iron(III) sulphate product formed in theabovementioned pressure leach step is composed of one or morejarosite-type minerals, such as hydronium, sodium, potassium or ammoniumjarosite. In one preferred embodiment of the present invention, thepH-stable iron(III) sulphate product is sodium jarosite or a solidsolution of hydronium and sodium jarosite.

While it is common for a metal value-bearing material containing arsenicand/or antimony such as a copper sulphide ore or concentrate containingarsenic and/or antimony to also contain at least trace amounts of ironcompounds, the inventors have advantageously found that the presence ofadditional iron compounds in the feed material subjected to the pressureleaching process also promotes the formation of copper-free secondaryferric sulphate minerals that also contain arsenic and/or antimony.Preferably, the molar ratio of Fe:(As+Sb) in the feed material to step(b) is greater than about 1:1 on a molar basis, preferably greater thanabout 2:1 and more preferably greater than about 4:1. Thus by ensuringthat the Fe:(As+Sb) ratio in the feed material to step (b) is greaterthan about 1:1, preferably greater than about 2:1 and more preferablygreater than about 4:1, the bulk of the arsenic and/or antimony in thefeed material reports to the residue as an environmentally stableiron-arsenate and/or iron-antimonate phase, rather than as acopper-iron-sulphate-arsenate/antimonate.

The inventors have found that the iron compounds suitable for theabovementioned modification to the Fe:(As+Sb) molar ratio of the feedmaterial are such compounds that are readily solubilised under theacidic high temperature/high pressure leach conditions of the invention.The particle size of the suitable iron compounds will typically be suchthat the solubilisation kinetics are compatible with the retention timein the high temperature/high pressure leach stage.

Provided that the requirement for rapid solubilisation under the hightemperature/high pressure leach conditions is met, the chemical valencyof the iron compounds added to the feed material to adjust theFe:(As+Sb) molar ratio to the required level is not thought to becritical. This is because, under the operating conditions of the hightemperature/high pressure leach step (b), substantially all ferrous iron[Fe(II)] will be rapidly oxidised to the ferric [Fe(III)] state. Inother words, the iron compounds may be ferrous or ferric compounds, ormixed ferrous/ferric compounds. However, it is preferred that the ironcompounds are in the ferric state since this reduces the oxygenconsumption during the high temperature/high pressure leach step.

In one preferred embodiment of the invention, the iron compounds arederived from pyrite, in particular calcined pyrite produced underconditions that favour the formation of FeS, FeO, Fe₃O₄ or gamma-Fe₂O₃over the formation of alpha-Fe₂O₃, since the former iron compounds aremore readily solubilised compared with the latter iron compound.

During the high temperature/high pressure leach step there are manycompeting chemical reactions relating to the formation and precipitationof different iron-containing species, such as, for example, basic ferricsulphates, hematite, and jarosite. Promotion of precipitation ofjarosite and/or hematite over basic ferric sulphate is favoured by thepresence of suitable chemical reactions that decrease the effectiveconcentration of free acid generated during the high temperature/highpressure leach step.

In one preferred embodiment of the invention, the chemical agents addedto the feed material being subjected to the high temperature/highpressure leach step comprise metal salts which directly participate inthe formation of jarosite-type compounds, in particular soluble alkalimetal ion salts such as those of sodium or potassium, and ammoniumsalts, all of which form stable jarosite-type minerals of the generalformula MFe₃(SO₄)₂(OH)₆ where M=Na, K and NH₄, respectively. Theformation of these jarosite-type minerals decreases the effectiveconcentration of free acid in the leach slurry under the prevailing hightemperature/high pressure leach conditions. The addition of such solublealkali metal ion salts also increases the temperature at which jarositetype minerals tend to form in preference to basic iron sulphate typeminerals during the pressure oxidation process at any given acidconcentration. The ability to operate at higher temperatures whilepromoting the formation of pH-stable iron(III) sulphate products overbasic iron sulphate type minerals provides economic advantages in theform of enhanced leaching reaction kinetics and shorter requiredresidence times

In another preferred embodiment of the invention, the chemical agentsalso comprise soluble sulphate salts whose cations are merely spectatorions and as such do not participate in any precipitation reactions. Thepreferred chemical agents particularly include magnesium and/or zincsulphate. Addition of a suitable soluble sulphate increases theconcentration of the bisulphate ion present in the high temperature/highpressure leach slurry and decreases the effective concentration of freeacid at temperature from that which would otherwise be experienced at agiven feed solids composition and concentration (% solids). The solublesulphate salts may be added directly to the high temperature/highpressure leach step or generated by reacting carbonate and/or hydroxidesalts of magnesium and/or zinc in the high temperature/high pressureleach step. In another preferred embodiment of the invention, thesoluble zinc salt may be introduced by the leaching of zinc sulphideminerals that may be present in the feed material.

In a further preferred embodiment of the invention, the chemicalreagents may also comprise bases or carbonates, in particular limestoneor lime, which directly consume acid and decrease the effectiveconcentration of free acid in the high temperature/high pressure leachstep.

In the case of copper sulphides containing arsenic and/or antimony,copper dissolution in the high temperature/high pressure leach step isoptimized by addition of iron compounds to the reaction vessel,typically an autoclave, in sufficient quantities to favour precipitationof environmentally stable secondary iron-arsenate and/or iron-antimonateand/or iron-arsenate-sulphate and/or iron-antimonate-sulphate phaseswithin the autoclave rather than the precipitation of copper-containingarsenate-antimonate residues, thereby limiting the copper content of theresidue and maximising the soluble copper content of the resultantliquid stream available for copper recovery by a combination of solventextraction and electrowinning or by means of another suitable recoverymethod.

By means of limiting the copper content of the solid leach residue andefficient separation of the soluble copper from the solid leach residue,the economics of precious metals recovery from the leach residue bycyanidation is enhanced as the extent of the reaction between copper andthe cyanide leachant is significantly reduced, thus lowering the overallcyanide consumption.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with various aspects of the present invention, a metalvalue-bearing material containing arsenic and/or antimony that alsoconstitutes a source of sulphate ions is provided for processing. Themetal value-bearing material may be an ore, a concentrate, or any othermaterial from which metal values, in particular copper and preciousmetals such as gold and silver, may be recovered. The present inventionis equally applicable to other metal value-bearing materials containingarsenic and/or antimony such as various ores and concentrates containingother valuable metals such as nickel, cobalt, zinc and theplatinum-group metals.

For convenience, however, the description of the preferred embodimentsof the invention is restricted to copper-containing materials that alsocontain arsenic and/or antimony. The copper-containing material ispreferably a copper sulphide ore or concentrate that contain arsenicand/or antimony, and particularly applies to ores and/or concentratesthat contain tennantite (Cu₁₂As₄S₁₃), enargite (Cu₃AsS₄) andtetrahedrite (Cu₁₂Sb₄S₁₃), and to other ores or concentrates containingcopper sulphide minerals such as, for example, chalcopyrite (CuFeS₂),chalcocite (Cu₂S), bornite (Cu₅FeS₄), covellite (CuS), when contaminatedwith arsenic- and antimony-bearing material.

Geologically, gold is frequently associated with metal sulphide oressuch as, for example, pyrite, chalcopyrite, galena, arsenopyrite andstibnite. Gold is also often present in sulphide concentrates producedfrom such ores. Accordingly, a preferred embodiment of the presentinvention is particularly advantageous in connection with the recoveryof copper and precious metals, such as gold, from mixed gold/copper oresor concentrates containing arsenic and/or antimony.

Thus, the metal value-bearing material is preferably a mixed gold/copperore or concentrate containing arsenic and/or antimony, and morepreferably, a mixed gold/copper sulphide ore or concentrate containingarsenic and/or antimony. Typically, the mixed gold/copper sulphide oreis a tennantite-enargite-chalcopyrite-pyrite ore.

The metal value-bearing material typically undergoes comminution,flotation, blending and/or slurry formation, as well as chemical and/orphysical conditioning to afford a feed stream which, in turn, issubjected to a high temperature/high pressure oxidative leach step and aseries of downstream unit stages to afford recovery of the containedmetal values.

The specific conditions applicable to the comminution, flotation andconditioning stages are determined by the chemical and physicalproperties of the metal value-bearing ore material. As a general rule,these specific conditions are designed to yield a concentrate thatoptimises recovery versus grade. These specific conditions do not have adirect bearing on the application of the preferred embodiments of thepresent invention. As such, the present invention is specificallyconcerned with the treatment of a dewatered concentrate exiting thecomminution, flotation and conditioning circuits.

The dewatered concentrate stream is repulped with water, preferablyprocess waters including but not limited to magnesium and/or zinccontaining solutions recycled from the copper recovery circuit,hereinafter referred to as neutral barren solution (NBS). The repulpedconcentrate stream may undergo an additional regrinding step to yield afiner particle size than the natural particle size produced from theflotation concentrator if desired to facilitate a reduction in theresidence time of the downstream high temperature/high pressure leachstep.

After the metal value-bearing concentrate stream has been suitablyprepared as a slurry, the slurry is fed to an agitated pressure vessel,preferably an autoclave, and subjected to pressure oxidation. Typicallythe high temperature/high pressure leaching process is carried out in asuitable pressure vessel, preferably an autoclave, at a temperature inthe range of from about 180° C. to about 250° C., preferably from about190° C. to about 230° C. The optimum temperature depends on manyfactors, including but not limited to, the mineralogical composition ofthe feed, the sulphide sulphur content of the feed, the particle sizedistribution of the feed, and the pulp density. As a general rule, thehigher temperatures in the above ranges provide for shorter retentiontimes and/or a reduction and/or elimination of the need for regrindingof the feed material prior to the high temperature/high pressure leachstep.

The high temperature/high pressure leaching process is typically carriedout a total pressure sufficiently high to provide an oxygen partialpressure inside the autoclave of between about 100 kPa and 1500 kPa,preferably in the range of from about 400-1000 kPa, taking into accountthe partial pressure of steam and other non-condensable gases within theautoclave such as nitrogen and carbon dioxide. Oxygen is typicallydelivered to the autoclave at a pressure above that inside the autoclaveand is introduced to the autoclave through spargers that discharge theoxygen gas beneath the autoclave agitators. The autoclave agitators aredesigned to maximise oxygen mass transfer from the gas phase to the feedslurry.

The high temperature/high pressure leaching step is typically conductedover a period of about 20 minutes to about 4 hours and more preferablyto about 2 hours, with higher operating temperatures and a finer feedparticle size facilitating shorter reaction times.

Under the high temperature/high pressure leaching process conditions,solid metal sulphide minerals within the feed material are oxidised tothe corresponding soluble metal sulphates. That is, the metal values arereleased into solution. The actual oxidation/dissolution reactions foreach metal sulphide mineral are a reflection of the chemical compositionof that mineral as well as the temperature and free acidity of the leachslurry, but the overall reaction can be simplified as shown in reaction(1).

MS(solid)+2O₂(gas)→MSO₄(solution)  (1)

The arsenic and antimony components of the feed material are oxidised tothe arsenate (AsO₄ ³⁻) and antimonite (SbO₄ ³⁻) species, respectively.

Some of the solubilised metal values then reprecipitate within theautoclave and report to the solid phase component on the autoclaveslurry as metal oxides and/or metal mixed hydroxyl-sulphates and/ormetal-sulphate-arsenate-antimonate species.

Iron may report to the solid phase component of the autoclave slurry asone or more different iron-containing compounds during the hightemperature/high pressure leach process, the identity of such phasesbeing determined by a specific set of operating conditions. For example,the formation of basic iron sulphate is favoured by high operatingtemperatures and high free acid concentrations. Under such conditions,the oxidation of pyrite (FeS₂), a significant component of many metalsulphide concentrates, can be represented by reaction (2).

4FeS₂+15O₂+6H₂O→4Fe(OH)SO₄+4H₂SO₄  (2)

The reaction of pyrite to form hematite (alpha Fe₂O₃) is favoured byhigh temperatures and low free acid concentrations according to reaction(3).

4FeS₂+17O₂+8H₂O→2Fe₂O₃+8H₂SO₄  (3)

The formation of jarosite is favoured by low operating temperatures andthe presence of cations such as Na⁺, K⁺ or NH₄ ⁺, according to reaction(4) where M=Na, K or NH₄.

12FeS₂+45O₂+30H₂O+2M₂SO₄→4MFe₃(SO₄)₂(OH)₆+18H₂SO₄  (4)

Hydronium jarosite, in which M=H₃O⁺, the hydronium ion of free acid,takes the place of Na, K or NH₄ is also favoured by low operatingtemperatures in the absence of such cations. Jarosite solid solutions inwhich M=varying proportions of Na, K, NH₄ and/or H₃O⁺ may also be formedby varying the quantity of Na, K, or NH₄ present.

Arsenate and antimonate species formed by the oxidation of the arsenicand antimony components of the feed material may precipitate as therespective iron(III) arsenate and iron(III) antimonate phases, but mayalso substitute for sulphate in, for example, the jarosite phase. Theprecipitation of arsenate as hydrated iron(III) arsenate, FeAsO₄.2H₂O,also known as scorodite, and the partial replacement of sulphate byarsenate in various jarosite phases is well documented in the scientificliterature. Jarosite is sometimes referred to as a scavenger for botharsenate and antimonate. The formation of hydrated iron(III) arsenateand/or arsenic-containing jarosite materials in the present invention isof considerable environmental benefit since these materials are known tobe environmentally stable and can be safely discharged into and storedin conventional residue storage impoundments.

Under typical prior art operating conditions for the hightemperature/high pressure leaching of mixed copper/gold metal sulphideconcentrates containing arsenic and/or antimony, formation of basic ironsulphate and hematite are favoured. The basic iron sulphate and hematitereport to the solid residue resulting from the high temperature/highpressure leach process. When the solid residue is washed, repulped andthen subjected to cyanidation in order to extract the precious metalvalues therein, there is an uneconomically high consumption of lime andcyanide. This is because the lime reacts directly with the basic ironsulphate during the adjustment of the pH to a value of 10 or higher thatis required for the precious metal cyanidation step.

In the present invention, additional iron compounds are added to thefeed material to the high temperature/high pressure leach step in orderto promote the formation of jarosite rather than basic iron sulphate.Under these conditions the jarosite phase acts as an efficient scavengerfor any soluble arsenate and/or antimonate formed during the pressureoxidation reactions. Moreover, the jarosite phase does not itself reactwith lime when the precious metals are recovered from the leach residueby cyanidation.

Preferably, the total iron content of the feed material added to thehigh temperature/high pressure leach process is such that the molarratio of Fe:(As+Sb) is greater than about 1:1, preferably greater thanabout 2:1 and more preferably greater than about 4:1. Apart fromfacilitating the formation of arsenic- and antimony-containing jarositephases which do not react with lime in cyanidation, the high Fe:(As+Sb)molar ratio reduces and/or prevents the formation and precipitation of amixed copper-iron-arsenate-antimonate-sulphate phase.

The iron compounds added to the metal value-bearing feed material inorder to adjust the molar ratio of Fe:(As+Sb) to the desired level areof a mineral/chemical composition and particle size such that they arereadily solubilised under the acidic high temperature/high pressureleach conditions.

The valency of the iron in the iron compounds is not thought to becritical because under the operating conditions of the hightemperature/high pressure leach process, substantially all iron(II) willbe oxidised to Fe(III). In other words the iron compounds may be ferrousor ferric compounds or mixed ferrous/ferric compounds, provided thatthey are soluble under the high temperature/high pressure leachconditions. However, it is preferred that the iron compounds arepretreated to maximize the ferric content and minimise any containedsulphide content in order to lower the overall oxygen consumptionrequired during the high temperature/high pressure leach step.

In a preferred embodiment of the present invention the iron compoundsare derived from pyrite, in particular calcined pyrite produced byoxidative conditions with the calciner operated in such a fashion as toproduce a calcined pyrite with a significant portion of the iron presentin a form readily capable of being solubilised in the autoclave underthe high temperature/high pressure leach conditions, such as forexample, FeS, FeO, Fe₃O₄ or gamma-Fe₂O₃, rather than alpha-Fe₂O₃produced in a conventional pyrite roaster, or the higher sulphidecontaining FeS₂ of uncalcined pyrite.

In a further embodiment of the present invention, the iron compounds maybe sourced from recycled process solutions containing iron sulphate,preferably in the ferric form, although the process solutions may alsocarry minor amounts of ferrous iron as well. Alternatively, the ironcompounds may be iron-containing precipitates from various other partsof the overall process, such as the iron-containing precipitate producedduring minor impurity removal ahead of or subsequent to metal valuerecovery steps such as copper recovery by a combination of solventextraction and electrowinning.

The iron compounds may be mixed with the metal value-bearing feed streambefore it is transferred to the high temperature/high pressure autoclaveleach vessel, or the iron compounds may be separately transferred to theautoclave before or after introduction of the feed stream to theautoclave.

One of the preferred embodiments of the present invention incorporatesthe addition of specific chemical agents which decrease the effectiveconcentration of free acid generated during the high temperature/highpressure leaching process thereby affording the precipitation of pHstable iron(III) sulphate compounds and avoiding the precipitation of abasic ferric sulphate. One group of chemical agents includes metal saltsthat directly participate in the formation of jarosite-type compounds,in particular sodium, potassium and ammonium jarosites. Such metal saltsinclude soluble alkali metal (sodium and potassium) and ammoniumsulphate. Typically the molar ratio of the added metal salt per mole ofiron present in the feed should be at least about 1:3 and preferably atleast about 1:2, that is, an excess of metal salt above thestoichiometric requirement.

Another group of chemical agents that have the ability to decrease theeffective concentration of free acid generated during the hightemperature/high pressure leaching process comprise soluble sulphatesalts whose cations are merely spectator ions and which do notparticipate in any precipitation reactions. Addition of soluble sulphateincreases the concentration of the bisulphate ion present at theoperating high temperature and decreases the effective concentration offree acid that would otherwise be experienced at the given temperature,feed solids composition and pulp density.

The soluble sulphate salts may be directly added to the hightemperature/high pressure leaching step or generated by reactingcarbonate or hydroxide salts of the appropriate metals. The inventorshave established that the appropriate metal sulphate salts include thoseof magnesium and zinc. Typically, magnesium is added as magnesiumcarbonate (magnesite), magnesium oxide, dolomite, or mixtures thereof.

The soluble sulphate salts, once added to or generated by the overallprocess, may be conveniently recycled in process water used for feedpreparation and/or autoclave quench water once the copper or otherdissolved metal values have been recovered from the leach solution.

The chemical agents may also comprise carbonates and other bases, inparticular limestone and lime, which directly consume acid and decreasethe effective concentration of free acid during the hightemperature/high pressure leaching process. Typically, bases are addedin an amount necessary to yield less than about 60 g/L sulphuric acid insolution in the product from the high temperature/high pressure leachstep, as measured by titration of slurry samples at ambient temperature.

The chemical agents may be mixed with the feed stream before it istransferred to the autoclave for the high temperature/high pressureleach step, or the chemical agents may be separately transferred to theautoclave before or after introduction of the feed stream to theautoclave.

During the high temperature/high pressure leaching step metal values, inparticular copper, may be solubilised to form a metal value-containingsolution. It is envisaged that the metal values will be recovered fromthe metal value-containing solution by well understood methods andtechniques. For example, where the metal value is copper, copper istypically recovered from the copper-containing solution by a combinationof solvent extraction and electrowinning. However, other metal recoveryprocesses such as cementation or precipitation of an intermediateproduct such as a hydroxide or sulphide could be employed.

Precious metal values, especially gold, contained in the feed materialwill report to the solid residue formed during the high temperature/highpressure leach process. It is envisaged that the precious metal valueswill be recovered from the solid residue by washing to remove entrainedacid and soluble metal values, repulping and treating the consequentslurry by a combination of conventional cyanidation, activated carbon,stripping, electrowinning and smelting techniques.

In a preferred embodiment of the present invention, it has been found tobe advantageous to maintain the slurry discharged from the autoclave ata temperature above about 70° C. and preferably in the range of fromabout 85-100° C. for a period in the range of from about 15 minutes toabout 4 hours in an agitated tank or tanks before it is subsequentlycooled to ambient temperature and subjected to solid/liquid separationby counter current decantation and thickening ahead of the preciousmetals recovery unit steps. This compares with prior art thatincorporates rapid cooling of the autoclave discharge slurry to ambienttemperature by means of a series of flash vessels and subsequentsolid/liquid separation processes generally conducted below about 70° C.The advantage of this slow cooling or digestion-conditioning stepdisclosed in the present invention relates to the fact that anyremaining basic ferric sulphate and/orcopper-iron-sulphate-arsenate-antimonate in the leach slurry will beconverted into a pH-stable iron(III) sulphate and/or redissolve, whichin the case of copper-iron-sulphate-arsenate-antimonate will releasesoluble copper, respectively. By this means, the lime consumptionrequired and, in the case of copper-containing feed materials, thecyanide consumption required for precious metal cyanidation should bereduced, while any copper losses to the solid leach residue should alsobe reduced.

By application of the preferred embodiments of the present inventioncopper recoveries in excess of 95% and lime consumption of less than 15kg/t of solid residue can be expected from a wide range of copper/goldsulphide ores and concentrates that also contain appreciable arsenicand/or antimony contents.

In summary, the advantages of the present invention compared with priorart include but are not limited by the following:

-   -   (a) enhanced recovery of metal values, typically copper and        precious metals, by preventing the coprecipitation of metal        values in the solid residue discharged from the high        temperature/high pressure step;    -   (b) prevention of the formation of unstable basic iron sulphate        species in the solid residue discharged from the high        temperature/high pressure step, or from the hot conditioning        step in a preferred embodiment, that consume excessive lime        during the recovery of the precious metals by cyanidation; and    -   (c) generate arsenic- and/or antimony-containing residues that        can be stored in conventional residue storage impoundments        without causing unacceptable environmental outcomes.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will now be furtherdescribed by reference to the following examples. The process conditionsreflected therein are intended to exemplify various aspects of theinvention, and are not intended to limit the scope of the claimedinvention. Numerous variations and modifications will suggest themselvesto persons skilled in the relevant art, in addition to those alreadydescribed, without departing from the basic inventive steps. All suchvariations and modifications are to be considered within the scope ofthe present invention, the nature of which is to be determined from theforegoing description.

Example

This example outlines the general scope of the preferred embodiments ofthe present invention as applied to the continuous processing of arun-of-mine tennantite-enargite-chalcopyrite-pyrite ore containing onaverage 1.7% Cu and 4.1 g/t Au. A simplified flowsheet of a preferredembodiment of the present invention is shown in FIG. 1.

After crushing and grinding, a copper concentrate typically containing16% Cu, 6% As and 28 g/t Au is produced by rougher, scavenger andcleaner flotation banks using the appropriate flotation reagent regime.The copper concentrate is directed to a copper concentrate dewateringcircuit where the free moisture content is reduced to about 10%.

A suitable pyrite concentrate, which may or may not also contain asignificant gold content, is transferred to a pyrite calcinationscircuit. The air and feed rates to the calciner as well as the operatingtemperature are controlled such that the formation of alpha-hematite inthe calciner product is minimised. In this way the iron content of thecalcined product is readily soluble in the subsequent hightemperature/high pressure leach step. Hot calcined pyrite is discharged,cooled and then combined with water to form a slurry.

The copper concentrate is repulped with neutral barren solution (NBS)that exits the impurity removal stage and is then subjected to aregrinding stage, if required, so that the particle size P₈₀ is lessthan about 50 micron, preferably about 38 micron, and more preferablyabout 25 micron.

The reground copper concentrate, and optionally iron-arsenic and/oriron-antimony residues generated during prior high temperature/highpressure leach cycles, is mixed with the required amount of calcinedpyrite in a suitable agitated autoclave feed storage tank such that theFe:(As+Sb) molar ratio of the autoclave feed is greater than about 1:1,preferably greater than about 2:1 and more preferably greater than about4:1.

Soluble salts of sodium and potassium and/or a suitable source ofmagnesium and/or zinc may be added individually or in combination to theautoclave feed storage tank in a slight excess of the stoichiometricamounts that are required to facilitate the precipitation of the bulk ofthe iron in the feed as sodium or potassium or hydronium jarosite,optionally in combination with hematite but without the formation ofappreciable basic iron sulphate, within the high temperature/highpressure autoclave while simultaneously ensuring that the free acidconcentration of the autoclave slurry is no greater than about 60 g/Land preferably about 40 g/L.

The combined slurry is directed to the first compartment of amulti-compartment high pressure autoclave fitted with a plurality ofagitators by means of a centrifugal pump feeding a positivedisplacement, piston driven diaphragm pump at an operating pressure ofpreferably over 300 kPa higher than the steam saturation pressure at theoperating temperature of the autoclave. This operating pressure willgenerally be over 2000 kPa.

High pressure steam is supplied to the autoclave for initial heat-up andon an as-needed basis.

Each compartment of the autoclave is fitted with a quench water systemby which a controlled flow of quench water can be directly injected intoeach compartment such that the desired operating temperature, typicallyin the range of from about 190-230° C., is continuously maintained.Alternatively, if required, each autoclave compartment can be equippedwith a suitable cooling coil, the flow of coolant being controlled inorder to maintain the desired operating temperature.

Oxygen at 94% or greater purity is delivered from a cryogenic oxygenplant to the autoclave by bottom entry spargers entering beneath theautoclave agitators at a pressure of greater than 2000 kPa. The bottomimpeller on the agitators is of the Rushton or Smith turbine design inorder to maximise oxygen mass transfer to the feed slurry.

Following the required retention time, typically 20 minutes to about 2hours, the processed slurry is discharged from the autoclave via asuitable flash vessel, cooled to a temperature of about 85-100° C. andstored at this temperature for a period of about 15 minutes to about 4hours in a suitable agitated digestion/conditioning tank. From there theconditioned autoclave slurry is subjected to solid/liquid separation viaa series of conventional counter current thickeners.

The thickened underflow is washed to remove entrained leach solution,filtered and the resultant cake forwarded to a conventional goldcyanidation circuit.

The thickener overflow contains the dissolved copper content of the feedand is directed to a partial neutralisation (PN) circuit as pregnantleach solution (PLS). As the PLS contains a relatively high sulphuricacid concentration, typically 30-60 g/L, excess acid is neutralised byaddition of a limestone slurry in order to achieve a final PLS freeacidity of about 2 to 5 g/L (pH approximately 1.5). After solid/liquidseparation to remove precipitated solids, principally gypsum, theneutralised PLS is clarified before the copper is recovered byconventional solvent extraction and electrowinning techniques.

The raffinate from the solvent extraction circuit is then subjected toan impurity removal (IR) step by addition of limestone and limeslurries. After removal of precipitated solids by thickening, theclarified neutral barren solution is used in a variety of appropriateduties, including repulping of the incoming dewatered copperconcentrate, filter wash water and as autoclave quench water.

If required, the precipitated solids from the partial neutralization(PN) and/or impurity removal (IR) steps may be recycled to the hightemperature/high pressure leach process as sources of iron-containingand/or acid consuming products.

In the preceding description of the invention and in the claims whichfollow, except where the context requires otherwise due to expresslanguage or necessary implication, the words “comprise” or variationssuch as “comprises” or “comprising” are used in an inclusive sense,i.e., to specify the presence of the stated features, but not topreclude the presence or addition of further features in variousembodiments of the invention.

It is to be understood that this invention and the preferred embodimentsare not limited to the particular materials described, as these mayvary. It is also to be understood that the terminology used herein isfor the purpose of describing particular embodiments only, and is notintended to limit the scope of the present invention in any way.

It is also noted that, as used herein, the singular forms of “a”, “an”and “the” include the plural unless the context clearly requiresotherwise. Unless defined otherwise, all technical and scientific termsherein have the same meanings as commonly understood by one of ordinaryskill in the art to which this invention belongs.

1. A method for the recovery of metal values from a metal value-bearingmaterial containing arsenic and/or antimony and a source of sulphateions, the process comprising the steps of: (a) providing a feed streamcomprising a metal value-bearing material containing arsenic and/orantimony and a source of sulphate ions; (b) subjecting the feed streamto oxidative conditions under elevated temperature and pressureconditions in the presence of at least one component selected todecrease the effective concentration of free acid generated during thepressure oxidation step and promote the formation of pH-stable iron(III)sulphate products, thereby forming a slurry comprising a metalvalue-containing leach solution and a solid leach residue containingpH-stable iron(III) sulphate products and environmentally stableiron-arsenic and/or iron-antimony products; (c) separating the metalvalue-containing leach solution from the solid leach residue; (d)recovering the metal value(s) from the metal value-containing leachsolution; and (e) recovering precious metal values, if present, in thesolid leach residue by cyanide leaching.
 2. A method according to claim1, wherein the slurry from step (b) is maintained at a temperature inthe range of from about 70° C. to about 100° C. for a period in therange of from about 15 minutes to about 4 hours prior to separating themetal value-containing solution from the solid leach residue.
 3. Amethod according to claim 1, wherein said metal value-bearing materialis a copper-bearing material containing arsenic and/or antimony and asource of sulphate ions.
 4. A method according to claim 1, wherein saidsource of sulphate ions is a sulphide ore or ore concentrate.
 5. Amethod according to claim 1, wherein said at least one componentselected to decrease the effective concentration of free acid generatedduring the pressure oxidation step and promote the formation ofpH-stable iron(III) sulphate products includes an iron-containingcomponent.
 6. A method according to claim 1, wherein said at least onecomponent selected to decrease the effective concentration of free acidgenerated during the pressure oxidation step and/or promote theformation of pH-stable iron(III) sulphate products further includes achemical agent.
 7. A method according to claim 5, wherein saidiron-containing component provides a source of soluble iron and is addedto the feed stream before pressure oxidation such that the molar ratioof iron to arsenic and/or antimony in the feed stream is greater thanabout 1:1, preferably greater than about 2:1 and more preferably greaterthan about 4:1.
 8. A method according to claim 5, wherein saidiron-containing component includes at least one component selected frompyrite concentrate, calcined pyrite concentrate and any recoverediron-containing residues formed during a pressure oxidation process. 9.A method according to claim 8, wherein said calcined pyrite concentrateis substantially free of alpha Fe₂O₃ and contains one or more compoundsselected from FeS, FeO, Fe₃O₄ and gamma Fe₂O₃.
 10. A method according toclaim 1, wherein said at least one component selected to decrease theeffective concentration of free acid generated during the pressureoxidation step and promote the formation of pH-stable iron(III) sulphateproducts includes a chemical agent.
 11. A method according to claim 6,wherein said chemical agent includes at least one material sourced fromlimestone or lime, or one or more soluble salts containing magnesium,sodium, potassium and/or ammonium.
 12. A method according to claim 11,wherein said chemical agent is added in an amount sufficient to promotethe formation of a jarosite phase containing sodium, potassium and/orammonium and to limit the formation of any pH-unstable or meta-stablesolid ferric sulphate products during pressure oxidation.
 13. A methodaccording to claim 11, wherein said one or more soluble salts containingmagnesium includes one or more compounds selected from magnesium oxide,magnesium hydroxide, magnesium carbonate (magnesite) andmagnesium-calcium carbonate (dolomite).
 14. A method according to claim1, wherein said at least one component is added to the feed stream priorto step (b).
 15. A method according to claim 1, wherein said feed streamis an ore or ore concentrate containing said metal values, and whereinsaid metal values include a base metal selected from the groupconsisting of copper, nickel, cobalt, zinc and platinum.
 16. A methodaccording to claim 1, wherein said feed stream is an ore or oreconcentrate containing said metal values, and wherein said metal valuesinclude a precious metal of gold and/or silver.
 17. A method for therecovery of metal values from a metal value-containing feed materialcontaining arsenic and/or antimony and a source of sulphate ions, themethod comprising the steps of: (a) providing a feed stream of metalvalue-containing material containing arsenic and/or antimony and asource of sulphate ions; (b) subjecting the feed stream to acidicoxidative conditions under elevated temperature and pressure conditionsin the presence of at least one iron-containing compound and/or at leastone chemical agent, wherein the iron-containing compound and/or chemicalagent is selected to decrease the effective free acid concentrationgenerated during the pressure oxidation step and promote the formationof pH-stable iron(III) sulphate products, thereby forming a metalvalue-containing solution and a solid leach residue containing pH-stableiron(III) sulphate products and environmentally stable iron-arsenicand/or iron-antimony products; (c) separating the metal value-containingsolution from the solid leach residue; (d) recovering the metalvalue(s), if desired, from the metal value-containing solution; and (e)recovering precious metal values, if present, in the solid leach residueby cyanide leaching.
 18. A method according to claim 17, wherein themetal value-containing solution and solid leach residue obtained fromstep (b) is maintained at a temperature in the range of from about 70°C. to about 100° C. for a period in the range of from about 15 minutesto about 4 hours prior to separating step (c).
 19. A process forrecovery of metal values from a metal value-bearing material containingarsenic and/or antimony and a source of sulphate ions, the processcomprising the steps of: (a) providing a feed stream comprising a metalvalue-bearing material containing arsenic and/or antimony and a sourceof sulphate ions; (b) subjecting the feed stream to oxidative conditionsunder elevated temperature and pressure conditions in the presence of atleast one iron-containing compound and/or at least one chemical agent,wherein the iron-containing compound and/or chemical agent is selectedto decrease the effective concentration of free acid generated duringthe pressure oxidation step and promote the formation of pH-stableiron(III) sulphate products, thereby forming a slurry comprising a metalvalue-containing leach solution and a solid leach residue containingpH-stable iron(III) sulphate products and environmentally stableiron-arsenic and/or iron-antimony products; (c) maintaining the slurryobtained from step (b) at a temperature in the range of from about 70°C. to about 100° C. for a period in the range of from about 15 minutesto about 4 hours; (d) separating the metal value-containing leachsolution from the solid leach residue; (e) recovering the metal value(s)from the metal value-containing leach solution; and (f) recoveringprecious metal values, if present, in the solid leach residue by cyanideleaching.
 20. A method for recovering one or more metal values from ametal value-containing feed material containing arsenic and/or antimony,the method comprising the steps of: (a) providing the feed material to apressure oxidation reactor; (b) subjecting the feed material within thepressure oxidation reactor to pressure oxidation in the presence of atleast one component selected to decrease the amount of free acidgenerated during the pressure oxidation and/or promote the formation ofpH-stable iron(III) sulphate products, to form a slurry comprising: (i)a metal value-containing solution; and (ii) a solid leach residuecomprising the pH-stable iron(III) sulphate products and environmentallystable iron-arsenic and/or iron-antimony products; (c) separating themetal value-containing solution from the solid leach residue; and (d)recovering the metal values from the metal value-containing solution.21. A method according to claim 1, wherein prior to the step ofrecovering the metal value(s) from the metal value-containing leachsolution, the pH is reduced to a pH of less than about pH
 2. 22. Apressure oxidation process for recovering one or more metal values froma metal value-bearing material containing arsenic and/or antimony,comprising the step of providing to the process at least one componentselected to promote the formation of solids that are sufficiently stableunder pressure oxidation conditions such that the recovery of the one ormore metal values is enhanced.
 23. A process according to claim 22,wherein said at least one component is selected to promote the formationof solids that do not significantly bind said one or more metal values.24. A process according to claim 22, wherein the at least one componentis selected to promote the formation of solids capable of bindingarsenic and/or antimony.
 25. A process according to claim 22, whereinsaid one or more metal values include a base metal and a precious metaland wherein said at least one component promotes the formation of solidsthat are stable under alkaline conditions of cyanidation processes usedto recover precious metal values following the recovery of base metalvalues.
 26. A process according to claim 22, wherein said at least onecomponent is an iron-containing compound.
 27. A process according toclaim 22, wherein said at least one component further includes at leastone chemical agent.
 28. A process according to claim 22, wherein said atleast one component is a chemical agent.
 29. A method for the recoveryof metal values from a metal value-containing feed material containingarsenic and/or antimony and a source of sulphate ions, the methodcomprising the steps of: (a) providing a feed stream of metalvalue-containing material containing arsenic and/or antimony and asource of sulphate ions; (b) subjecting the feed stream to acidicoxidative conditions under elevated temperature and pressure conditionsto form a metal value-containing solution and a solid leach residue; (c)maintaining a temperature in the range of from about 70° C. to about100° C. for a period in the range of from about 15 minutes to about 4hours prior to separating step (d) to promote the formation of pH-stableiron(III) sulphate products; (d) separating the metal value-containingsolution from the solid leach residue; (e) recovering the metalvalue(s), if desired, from the metal value-containing solution; and (f)recovering precious metal values, if present, in the solid leach residueby cyanide leaching.